We are following the production of the parvovirus, KRV, as a tool to understand both cellular and viral mRNA synthesis. Viral specific RNAs are synthesized in the nucleus, processed, modified and transported to the cytoplasm, presumably by the same mechanism as cellular mRNA biosynthesis. However, viral RNA synthesis involves fewer species of synthesized RNA than total cellular RNA. Hence, it is a simpler system for study than the complex eukaryotic cellular RNA. The viral RNAs can be identified as viral specific by hybridization, gel electrophoresis, restriction enzyme analysis and by Northern and Southern blotting techniques. We have isolated and partially characterized at least five KRV specific RNAs in the infected rat nephroma cell. These RNAs are probably the functional messages. The longest KRV RNA is 4.7 Kb and would represent a transcript of 95 to 100% of the viral genome. The most abundant message, about 3.0 Kb, represents over 50% of the viral genome and probably codes in the reticulocyte transcribing system for the most abundant viral protein (mw 68,000) which is the main viral capsid protein. This abundant RNA, on the basis of R loop data, has an origin of transcription about 0.38 to 0.42 map units from the 3' end of the single stranded KRV genome.